Message transmitters at work

Using a single-molecule microscopy, an international team of researchers has observed live for the first time G protein-coupled receptors that perform critical functions such as mediating the biological effects of hormones. Their findings, published in the science journal Nature, could help in the search for new drugs.

When your heart beats faster during states of excitement, G protein-coupled receptors (GPCRs) have no small part to play in this. That’s because these sensors on the cell membrane ensure that hormones like adrenalin and other signaling molecules can pass on messages from a cell’s surroundings to its interior. Only when every cell knows what’s happening around it can billions of cells work in harmony. Without these receptors, lots of basic bodily functions would not be possible.

As their name suggests, these receptors have to “find” and interact with so-called G proteins on the cell membrane in order to initiate and regulate intracellular processes. How and where this happens has long been the subject of numerous hypotheses that scientists have thus far been unable to confirm.

Using a highly sophisticated method based on a two-color, single-molecule microscopy, a research team led by Prof. Martin Lohse at the University of Würzburg and the Max Delbrück Center for Molecular Medicine (MDC) in Berlin has now succeeded for the first time in directly observing and investigating the initial contacts between individual receptors and G proteins as well as what takes place next at the surface of living cells. Researchers at the Wroclaw University of Science and Technology contributed by developing mathematical models for these interactions. The research team recently published its findings in the journal Nature.

Hot spots for message transmission

Receptors and G proteins prefer to interact at special sites on the plasma membrane. “We call these hot spots,” says Prof. Davide Calebiro, the study’s senior author, who is moving from the University of Würzburg to take up a professorship at the University of Birmingham in England. To understand these previously unknown interaction points, researchers looked at other structures underneath the cell membrane – such as the cytoskeleton. They showed that barriers like straw-shaped filaments (microtubules), actin fibers, and clathrin-coated vesicles play a role in the formation of the hot spots.

“We believe that the hot spots influence G protein activation, making the whole process faster and more efficient,” says Dr. Titiwat Sungkaworn, the study’s first author. At the same time, the hot spots ensure that signal transmission can be limited to small areas within the cell. Another finding is that receptors and G proteins usually remain associated only for a short time. Indeed, most interactions ended after just one second. These brief interactions are generally unproductive in that they fail to bring about the forwarding of a signal. However, as soon as hormones or active pharmaceutical agents activate the receptors, the number of productive interactions increases and a signal is transmitted into the cell interior.

GPCRs are key molecular targets for drug discovery

Fundamental biological processes, the researchers stressed, can be highly sophisticated when observed up close. They anticipate that the current “extraordinary advances” in microscopy will shed entirely new light on the transmission and processing of biological signals.

These new insights could lead to the development of new therapeutic approaches. GPRCs are already among the most important molecular targets for drug discovery. About one-third of all pharmaceuticals act on the GPRC superfamily of proteins, which consists of more than 800 members. These receptors are viewed as especially promising targets in the search for new ways to treat high blood pressure, asthma, or Parkinson’s disease.

Research continues at the MDC

“Active agents have thus far either blocked or activated these receptors,” says Prof. Martin Lohse, head of the research team and scientific director of the MDC. In the future, it might be possible to fine-tune the signal transmission – for example, by having active agents influence the mobility of receptors and G proteins on the cell membrane or their interactions at the hot spots.

His research team at the MDC will continue to investigate the temporal and spatial aspects of GPCR activation. The researchers, for example, seek to describe the subsequent signaling steps concurrently and in other color channels – even below the resolution limit in light microscopy. This will require combining single-molecule microscopy with high-resolution procedures. Innovative new microscopes that meet these special needs are already up and running.

Photo: Simultaneous visualization and tracking of individual receptors (green) and G proteins (magenta) at the surface of a living cell. The receptors and G proteins undergo transient interactions, which occur preferentially at “hot spots” on the cell membrane. Credit: Lohse Lab

This study was supported by the German Research Foundation (DFG), the European Research Council (through an ERC Advanced Grant), and the National Science Centre in Poland.